preliminary analysis of size frequency and CPUE for the Indian Ocean bigeye tuna (Thunnus obesus) using Chinese longline observer data

This study presents the spatial pattern of length frequency and catch-per-unit-effort (CPUE) for the Indian Ocean bigeye tuna (Thunnus obesus) based on the Chinese longline fishery observer data from 2012 to 2019. Contour lines map and G-statistic method was used to make spatial distribution and autocorrelation analysis of size data, respectively. The standardization of CPUE (annual series as well as quarterly series in number/1000 hooks) was conducted by Generalized Additive Model (GAM) with variables including: year (year-quarter), latitude, longitude, area, target species, and depth of hooks. Spatial strata were defined by an adaptive area stratification used in the previous study. The results of spatial length frequency showed the large size groups (fork length greater than 110 cm) were mainly caught in the Northwest Indian Ocean; while the small size groups (fork length less than 110 cm) were more caught in the Southwest Indian Ocean. Both standardized indices indicate an overall decline, except for an increase in 2017 and decreased again after that. CPUE index mainly distributed in the medium-size group.